The human body isn't as strong as people think.
Friend of mine has had 2 heart attacks, one aged 20 and another at 22. He's always kick the shit out of it though, binge drinking, coffee, red bull and smoking.
When you're young you sometimes feel invincible, but the truth is far from it.

Glad you're ok though, never a nice feeling when the docs make urgent calls to the ambulance. Time to make some changes now you have your second chance.